Generally, a circularly welded joint obtained by welding the ends of two pieces of steel plates perpendicularly combined together has been much used as a welded joint for buildings, ships, bridges, construction machinery and off-shore structures, and a variety of welding methods have also been employed, such as arc welding, plasma welding, laser welding, electron beam welding and the like.
The circularly welded joint receives repetitive load due to wind, waves and mechanical vibration, and it is very important to improve its fatigue strength. As methods of improving the shapes of the welded beads and the fatigue strength by the treatment after the welding, there have been employed (1) grinding, (2) TIG dressing, (3) shot peening and (4) hammer peening accompanied, however, by the problems described below.
Here, the grinding and the TIG dressing are for improving the shapes of the welded beads, both of which, however, are very inefficient.
Shot peening and the hammer peening are effective in improving the fatigue strength. However, shot peening requires a huge machine as well as various utilities.
Further, hammer peening is accompanied by a large reaction and poor stability in the result of treatment often causing the press formability and the fatigue strength to be rather decreased. Further, hammer peening gives too great a plastic deformation and cannot be favorably applied to thin plates.
Moreover, grinding and hammer peening subject the joint to machining at a frequency as low as several hertz. Therefore, the machined surface becomes very rough. If stress concentrates in the mountain portions and if a load is repetitively exerted on the joint, cracks develop in the portion where stress is concentrated resulting in a decrease in the fatigue strength of the joint as a whole.
Generally, further, residual stress is introduced into the welded portion due to the heat input of welding. The residual stress is one of the factors that decrease the fatigue strength in the welded portion. As another means for improving the fatigue strength, therefore, there has been known a method of increasing the fatigue strength by producing compressive residual stress in the welded joint or by decreasing the tensile residual stress that is generated in the welded joint.
For example, compressive residual stress can be imparted by effecting the shot peening near the welded end. Here, the shot peening is a method of imparting the compressive residual stress by striking a number of steel balls of sizes of not larger than 1 mm onto a portion where cracks occur due to fatigue.
It has further been known that the shape of the welded end can be improved or the tensile residual stress can be decreased by heating and melting again the welded metal.
However, the shot peening requires steel balls posing problems of after treatment with steel balls and cost. There, further, exists a problem of dispersion in the margin for improving the fatigue strength.
As described above, the conventional technology for improving the fatigue strength cannot be employed for the circularly welded joint. Even if it could be employed, the margin of improving the fatigue strength is confined to a low level.
As prior art related to a method of improving the fatigue strength by applying ultrasonic oscillation to the welded joint, for example, U.S. Pat. No. 6,171,415 discloses a method of applying ultrasonic oscillation along the weld-seamed portion heated by the arcing of welding.
According to this prior art, however, it is a prerequisite to impart ultrasonic oscillation to a material heated at a high temperature immediately after welding and, besides, there has been disclosed no concrete range for impact with ultrasonic oscillator.
In order to improve the fatigue strength of the welded structure, further, there have been developed steel plates for suppressing the propagation of cracks due to fatigue and a variety of proposals have heretofore been made.
For example, according to the 24th Proceedings of the Fatigue Symposium, “Fatigue Properties of the Surface Layer Ultra-Fine Granulated Steel Plate”, Japanese Academy of Materials, 1998, pp. 157-162, there has been disclosed that a so-called SUF steel forming an ultra-fine microstructure in the surface layer by working ferrite in a step of elevating the temperature of a steel material for general shipbuilding in the column of the kind of steel a in Table 1, exhibits the effect of delaying the propagation rate of cracks due to fatigue.
Further, JP-A-6-271985 discloses a steel plate which lowers the rate of propagation of cracks due to fatigue by effecting water-cooling after the two-phase zone rolling for lowering the finish rolling temperature of a steel plate which contains components shown in the column of the kind of steel b in Table 1, so that there forms Martensite-Austenite constituent in which the cracks, due to fatigue, undergo branching, making it possible to lower the propagation rate of cracks due to fatigue.
Further, JP-A-11-1742 discloses a steel plate for suppressing the propagation of cracks by controlling the form of the second phase in a composite microstructure comprising ferrite and second phase in a steel plate which contains components shown in the column of the kind of steel c in Table 1, and controlling the hardness of the ferrite and of the second phase, so that there occurs fine cracks from the main cracks in the second phase, which work to disperse and weaken the propagation of cracks.
JP-A-7-90478 discloses a steel plate which suppresses the propagation of cracks by rolling the steel plate which contains components shown in the column of the kind of steel d in Table 1 in the non-recrystallized zone, followed by slow cooling to form a γ-zone in which carbon is concentrated and, thereafter, effecting the quick cooling to control the formation of Martensite-Austenite constituent.
Further, JP-A-2002-129181 discloses a steel plate which suppresses the propagation of cracks due to fatigue by dispersing ferrite and second phase that has a strength greatly different from that of the ferrite so as to exist in suitable sizes and in suitable amounts in a steel plate which contains components shown in the column of the kind of steel e in Table 1 and, further, enabling a particular set microstructure to develop concurrently.
Further, JP-A-8-225882 discloses a steel plate which delays the rate of crack propagation by forming a steel plate containing components shown in the column of the kind of steel f in Table 1 and having a two-phase microstructure of ferrite and bainite, and by specifying the ratio of the ferrite phase portion, hardness of the ferrite, and number of phase boundaries between ferrite and bainite to lie within predetermined ranges.
Moreover, JP-A-11-310846 discloses a steel plate which renders the cracks, due to fatigue, to become stagnant by forming a steel plate containing components shown in the column of the kind of steel g in Table 1 and having a two-phase microstructure of ferrite and bentonite or a three-phase texture of ferrite, bainite and martensite, wherein, when a difference in the hardness between the microstructures among the composite microstructures is set to be greater than a predetermined value or when an average particle size of a soft portion or an average gap in a hard portion is suppressed to be smaller than a predetermined value in addition to the above, the plastic deformation is suppressed at the end portion in case the crack that is developing has arrived at the vicinity of the boundary between the hard portion and the soft portion.
However, even the above steel plates that suppress the propagation of cracks, due to fatigue, exhibit little effect for improving the fatigue strength in the presence of the tensile residual stress due to the heat input of the circularly welding.
That is, the stress concentrates at the welded end, and the concentration of stress is promoted if the tensile residual stress works on the end portion due to the heat input at the time of welding, causing a decrease in the fatigue strength to a conspicuous degree.